The IOUs are permanently valid, but the effective litigation period of IOUs is two years, and the IOUs typed by two people are as valid as those typed by a third person.

The legal basis is as follows:

Article 135 of the General Principles of Civil Law of People's Republic of China (PRC): The limitation period for requesting the people's court to protect civil rights is two years, unless otherwise stipulated by law.

Article 137 of the General Principles of the Civil Law of People's Republic of China (PRC): The limitation of action shall be counted from the time when one knows or should know that one's rights have been infringed. However, if more than 20 years have passed since the right was infringed, the people's court will not protect it. Under special circumstances, the people's court may extend the limitation of action.

Item (4) of Article 62 of People's Republic of China (PRC) Contract Law: If the time limit for performance is not clear, the debtor may perform it at any time, and the creditor may also request performance at any time, but the other party shall be given necessary preparation time.